gaming on 2 monitors will be very uncomfortable, as the center of the screen wil be between the two monitors.
if your gaming on 1 monitor, with the other as a secondary monitor, id imagine youd get around 60fps on high/ultra, and around 40/50 fps on ultra, depending on what games your playing.
by "2xGeForce GTX 780 Ti" you mean sli'd right? if so my numbers are somewhat accurate. heres a benchmark:
http://www.eteknix.com/4k-gaming-showdown-amd-r9-290x-crossfire-vs-nvidia-gtx-780-ti-sli/5/